This invention relates to a process for preparing a reduced surface area shaped alumina catalyst that maintains a high level of piece crushing strength. The catalyst is prepared using a dry high temperature calcination.
The controlled adjustment of a significant hydrocarbon conversion catalyst property of surface area has been found to be possible along with the maintenance and even improvement of another significant catalyst property of piece crushing strength. Surface area can allow acidic and metal supported reactions to occur, while piece crush strength permits catalyst particles to maintain their integrity, and thus their useful life.
U.S. Pat. No. 3,920,615 (Huang) discloses a calcination treatment of at least 800xc2x0 C. which is used to reduce the surface area of an alumina catalyst to between 10 m2/gm and 150 m2/gm. The catalyst displays improved selectivity in a process for long chain mono-olefin dehydrogenation from paraffins as part of the production of alkylaryl sulfonates. No mention is made to the resulting piece crushing strength from the procedure.
Canadian Patent No. 1,020,958 (Masologites) discloses a catalyst consisting of at least one platinum group component used in a reaction zone with a hydrocarbon and hydrogen under conditions causing coke deposition on the catalyst. It is regenerated by oxidation and the process repeated until the surface area is between 20 and 90% of the original value. The catalyst is then treated to incorporate at least one promoter metal selected from the group of Re, Ge, Ir, Sn, Au, Cd, Pb, rare earths, or a mixture thereof. The resulting catalyst shows increased stability in use thus requiring less frequent regeneration or replacement. Again no mention is made as to the resulting piece crushing strength from this procedure.
Applicants have found that piece crushing strength is a very important property for catalysts. This has been recognized in the art pertaining to hydrotreatment as disclosed in U.S. Pat. Nos. 4,767,523 and 4,820,676 (Kukes et al.) where a solution of ammonium sulfate is used to treat alumina such that after calcination its strength is increased when measured under high pressure fixed bed hydrotreating conditions.
Piece crushing strength is an even more important property for moving bed applications. When catalyst particles are moving through a reaction zone, higher piece crushing strength leads to less catalyst attrition and deterioration to fines. Catalysts with poor strength more often fracture, generating dust and catalyst fines that can become trapped against reactor screens. This can lead to blocked flow of reactants and products, which often may require a reforming unit to shut down for screen cleaning. Many commercial moving bed systems require catalyst make up in order to replace catalyst inventory lost to fines, dust, or cracked chips.
U.S. Pat. No. 5,552,035 (Potter et al.) discloses a method for hydro-thermally calcining an extruded bound zeolitic catalyst that can be used in a fixed bed reforming process, where calcination improves catalyst strength. In contrast to Potter et al., applicants have found that dry calcination gives even better %, retention of catalyst strength. By studying the controlled use of steam as part of state-of-the-art Potter et al. disclosed hydro-thermal calcination evaluation, a surprising result was obtained by removing the disclosed 30 volume % to 100 volume % water from the calcination atmosphere. In fact it was found that this water was causing substantial loss of piece crushing strength in achieving a desired reduction in catalyst surface area. By conducting a calcination at substantially dry conditions such that the moisture level remains less than 4 mass %, and preferably less than 3 mass %, an excellent combination of piece crushing strength and reduced surface area was obtained.
U.S. Pat. No. 4,483,693 (White et al.) discloses a process for steam reforming of hydrocarbons in the presence of greater than 1 ppm sulfur using a catalyst comprising an alumina with a surface area from about 30 to about 160 m2/g formed by calcination of pure single phase boehmite. No information is provided regarding calcination water content or catalyst strength.
It is an object of this invention to provide a process for preparation of a novel and useful catalyst with reduced surface area and high piece crushing strength.
A broad embodiment of the present invention is a process for preparing a shaped catalyst comprising an alumina support where the catalyst is treated with a dry high temperature calcination at a time and temperature sufficient to produce a catalyst with a surface area from about 150 m2/gm to about 180 m2/gm and a piece crush strength of at least 40 N/mm. This amounts to a surface area reduction from about 5% to about 30% of the original support as analyzed prior to the dry high temperature calcination with a concomitant maintenance of piece crush strength of greater than 95% of the original support as analyzed prior to the dry high temperature calcination. The catalyst is characterized with a X-ray powder diffraction pattern such that the ratio of peak intensities at respective two-{circumflex over (xe2x88x92)} Bragg angle values of 34.0:32.5 is at least about 1.2 and the ratio of peak intensities at respective two-{circumflex over (xe2x88x92)} Bragg angle values of 46.0:45.5 is at most about 1.1.
The calcination conditions comprise a temperature from about 700xc2x0 C. to about 900xc2x0 C., a moisture level of less than 4 mass % steam and a time limit of about 15 minutes to about 20 hours. A preferred calcination condition comprises a temperature from about 800xc2x0 C. to about 900xc2x0 C., a moisture level of less than 3 mass % steam and a time limit of about 30 minutes to about 6 hours.
Optionally, the catalyst has at least one platinum group metal dispersed thereon along with a halogen component, especially chlorine, and perhaps an additional element such as tin.
The preferred form of alumina is substantially gamma phase alumina based on high purity pseudoboehmite alumina powder or alumina hydrosol. The preferred shape is either substantially cylindrical or substantially spherical.
The product prepared by this process is preferably used in a moving bed catalytic reforming process for converting hydrocarbons in the presence of less than 1 ppm sulfur.
These as well as other objects and embodiments will become apparent upon a reading of the following detailed description of the invention.